1. Field of the Invention
The present invention relates to transmitters used in industrial process control systems and more particularly to the magnetic actuation of the zero and span adjustments of such transmitters.
2. Description of the Prior Art
Two-wire transmitters (as well as three-wire and four-wire transmitters) find widespread use in industrial process control systems, A two-wire transmitter includes a pair of terminals which are connected in a current loop together with a power source and a load, The two-wire transmitter is powered by the loop current flowing through the current loop, and varies the magnitude of the loop current as a function of a parameter or condition which is sensed, Three and four wire transmitters have separate leads for supply current and outputs, In general, the transmitters comprise energized electrical circuits which are enclosed in a sealed housing such that ignition of any combustible atmosphere by faults or sparks from the energized circuit is contained in the housing.
Although a variety of operating ranges are possible, the most widely used two-wire transmitter output varies from 4 to 20 milliamperes as a function of the sensed parameter, It is typical with a two-wire transmitter to provide adjustment of the transmitter so that a minimum or zero value of the parameter sensed corresponds to the minimum output (for example a loop current of 4 milliamperes) and that- the maximum parameter value to be sensed corresponds to the maximum output (for example 20 milliamperes).
The minimum and maximum parameter values will vary from one industrial process installation to another. It is desirable, therefore, to provide some means for setting the maximum and minimum output levels in the field, and this is done typically with electrically energized zero and span potentiometers sealed in the housing. With some transmitters, a housing cover must be removed to gain access to the potentiometers for adjustment, undesirably exposing the atmosphere surrounding the transmitter to the live circuits in the transmitter.
A variety of techniques are available for adjusting the potentiometers while sealing potentially explosive atmospheres surrounding the transmitter from the electrically live circuits in the transmitter. In some transmitters, a rotary adjustment shaft for adjusting a potentiometer is closely fitted through a bore in the housing to provide a long flame path for quenching ignition in the housing before it reaches the atmosphere surrounding the housing. In yet another arrangement, the potentiometers are mechanically coupled to a relatively large bar magnet which is then rotated magnetically by another bar magnet outside the live circuit's enclosure. This arrangement with bar magnets can have the disadvantage of mechanical hysteresis, making precise span and zero setting difficult. Actuated switches are also used for setting span and zero in transmitters, such switches requiring an opening through the wall of the transmitter's housing to provide for mechanical coupling to the switch.
For many process control environments, the transmitter itself is required to have an explosion-proof enclosure. This means that, if a spark takes place inside of the transmitter housing which ignites gases within the housing, no hot gases should be propagated from the interior of the transmitter to the exterior which could cause any surrounding combustible atmosphere to ignite.
Providing for zero and span adjustments which are accessible from outside the transmitter (so that the housing would not have to be opened) is desirable, but makes it difficult (or expensive) to maintain the explosion-proof characteristics of the transmitter. One arrangement for adjusting the zero and span of a transmitter from outside of the housing is suggested in U.S. Pat. No. 4,783,659 ("the '659 Patent") which issued on Nov. 8, 1988. The transmitter described in the '659 Patent includes a communications circuit which can take a variety of forms including, as is shown in FIG. 1 of the '659 Patent, magnetically actuated reed switches which are activated with a magnet from outside of the transmitter. The '659 Patent does not further show or describe the magnet or any structure for using the same to activate the reed switches.
In addition to the actuator disclosed in the '659 Patent, other external span and zero actuators have, in the past, needed either bulky magnet pairs for transmitting rotational force or passages formed through the transmitter housing wall, so that one end of the actuating mechanism extends into the chamber which contains the transmitter electronics, while the other end is accessible from the exterior of the transmitter. In order to maintain explosion-proof characteristics, very long flame paths must be created with very tight tolerances. It is also important that the passages be sealed so that moisture cannot enter the transmitter housing through the span and zero actuator passages.
As can be appreciated from the above, it would be desirable in providing for zero and span adjustments which are accessible from outside of the transmitter housing to eliminate the need for a long flame path and very tight tolerances. A transmitter which has externally accessible zero and span adjustments without the need for a long flame path and very tight tolerances is described in International Application Number PCT/US88/03280 which was published on May 5, 1989 as International Publication Number WO89/04014 ("the 03280 PCT application").
The transmitter described in the 03280 PCT application has zero and span magnetically actuated reed switches located in an interior chamber of the housing adjacent the housing's center wall. A relatively flat surface on the exterior of the transmitter housing has a recess formed therein. A pair of internally threaded blind holes extend downward from the recess into the center wall of the housing. A movable permanent magnet is situated in each blind hole. Each magnet is press fit into a lower recess of an associated screw which extend down into the associated blind hole. A spring is coaxially mounted on each magnet. A rubber washer is positioned below the head of each screw to provide an environmental seal for the blind hole. Access to the screws from the exterior of the housing is provided by a plate which is removably attached to the flat surface by a pair of screws.
Adjustment of the zero and span settings for the transmitter described in the 03280 PCT application is accomplished by first removing the plate with a screwdriver to thereby allow a technician to have access to the upper ends of the screws associated with the zero and span movable magnets. The technician can then reset the zero and span settings of the transmitter by using a screwdriver to loosen the screws. The spring associated with the screw is under compression and the loosening of the screw allows the spring to push the screw up so that the centerline of the magnet is aligned with the centerline of the associated reed switch. The electronics to which the reed switches are connected then adjusts the zero or span settings of the transmitter. After adjusting the zero and span settings of the transmitter the technician should tighten the screw to recompress the spring and move the centerline of the magnet out of alignment with the centerline of the reed switch. In addition, the technician should reattach the plate to the flat surface.
While the transmitter described in the 03280 PCT application does eliminate the need for a long flame path and very tight tolerances, it does not limit access to the movable magnets to only the personnel trained to perform the zero and span adjustments. The magnets are accessible to any individual who has access to the transmitter and a screwdriver. This makes the adjustment of the zero and span setting of the transmitter subject to tampering.
According to the 03280 PCT application the adjustment of the zero and span setting of the transmitter described therein may be made resistant to tampering by removing the screws and magnets as well as the associated return springs and rubber washers from the housing. The screws, magnets, return springs and rubber washers are relatively small parts and may be easily lost or misplaced if removed from the blind holes. As described above, the rubber washers provide an environmental seal for the blind holes. The rubber washer does not provide an environmental seal for the moving parts of the zero or span adjustment mechanism during the adjustment of the zero or span settings because the washer is moved away from its sealing face when the screw is moved. Use of the adjustment mechanism may then allow environmental contaminants to accumulate in each blind hole. The accumulated environmental contaminants may cause a malfunction of the moving parts. Removal of the washers may expose the internal threads of the blind holes to conditions which may make it difficult to loosen and tighten the screws (and therefore adjust the zero and span settings of the transmitter) when the screws are reinserted into the holes.